Heretofore, in a photolithographic technology, an exposure apparatus has been widely utilized which transfers a fine circuit pattern on a wafer to produce an integrated circuit. Along with high integration and high functionality of an integrated circuit, an integrated circuit becomes finer, and an exposure apparatus is required to form an image of a circuit pattern with high resolution on a wafer with a long focal depth, and shortening of the wavelength of the exposure light source is being advanced. The exposure light source has been advanced from conventional g-line (wavelength: 436 nm) or i-line (wavelength: 365 nm), and now, a KrF excimer laser (wavelength: 248 nm) or an ArF excimer laser (wavelength: 193 nm) is being used. Further, in recent years, use of an ArF excimer laser (wavelength: 193 nm) employing a liquid immersion technique has been under study.
The optical member to be used for an optical device employing such a light source, is required to be such that:
(1) the refractive index distribution An in an ultraviolet region is small,
(2) the change in absolute refractive index (compaction or rarefaction) by ultraviolet irradiation is small, i.e. the ultraviolet durability is high, and
(3) the birefringence is small.
Conventional synthetic quartz glass is known to undergo a phenomenon so-called compaction or rarefaction when irradiated with a high energy beam emitted from a light source such as a KrF excimer laser or an ArF excimer laser.
Compaction is a phenomenon wherein by the ultraviolet irradiation, the density of the irradiated portion of synthetic quartz glass increases, and along with this change in the density, the refractive index of the irradiated portion of synthetic quartz glass increases. On the other hand, rarefaction is a phenomenon wherein by the ultraviolet irradiation, the density of the irradiated portion of synthetic quartz glass decreases, and along with this change in the density, the refractive index of the irradiated portion of synthetic quartz glass decreases.
Which one of the phenomena i.e. compaction or rarefaction takes place, depends on the type of synthetic quartz glass and irradiation conditions (energy density and cumulative irradiation energy), and the phenomenon is complex.
Further, with synthetic quartz glass of the same composition, as the pulse energy density of irradiated light is high, the synthetic quartz glass tends to show compaction, and as the pulse energy density is low, the synthetic quartz glass usually tends to show rarefaction (Chris Van Peski, “Behavior of Fused Silica Under 193 nm Irradiation”, International SEMATECH Technical Report#00073974A-TR, (U.S.A.), International SEMATECH, Jul. 25, 2000).
In a semiconductor exposure device, a fine pattern on a photomask will be reduced and projected on a wafer. Accordingly, a projector lens is required to have an extremely high homogeneity of refractive index. If the refractive index of the projector lens is entirely or locally changed by ultraviolet irradiation, such a change presents an adverse effect such that the focal position becomes out of alignment, whereby the desired reduced projection of a pattern will be impossible.
Further, if the density of the irradiated portion of synthetic quartz glass is changed by ultraviolet irradiation, a stress will be induced at the irradiated portion and its vicinity, whereby the birefringence of synthetic quartz glass will be changed. This change in the birefringence will also have an adverse effect on the image forming performance of the projector lens, and thus is problematic.
The cause for the change in the density of synthetic quartz glass due to ultraviolet irradiation, has not yet clearly been understood, but some methods for improvement have been proposed.
For example, a method for producing synthetic quartz glass having a small degree of compaction, has been proposed (JP-A-2000-191329). The production method disclosed in this publication comprises heat treating in an oxidizing atmosphere a porous quartz glass body prepared by a VAD method, followed by transparent vitrification, and then heat treating in a hydrogen gas-containing atmosphere the transparent vitrified synthetic quartz glass block, to dope hydrogen molecules into the quartz glass.
The synthetic quartz glass obtained by this method contains no oxygen deficient type defects (≡Si-Si≡) and the degree of compaction by ultraviolet irradiation is small. By this method, it is possible to obtain synthetic quartz glass excellent in compaction without impairing other properties such as the homogeneity of refractive index. However, even the synthetic quartz glass produced by this method may sometimes shows rarefaction depending upon the irradiation conditions of ultraviolet lights, whereby the change in the refractive index of synthetic quartz glass will be large. Thus, the change in the refractive index of the synthetic quartz glass by ultraviolet irradiation has not yet been necessarily satisfactory.